1. Field of the Invention
The present invention relates to an image forming apparatus provided with a fixing unit for permitting a sheet bearing a toner image to pass between a heating member and a pressing member to heat and melt unfixed toner and fix it to the sheet.
2. Description of the Related Art
In recent years, attention has been focused on belt-type image forming apparatuses, in which a smaller heat capacity can be set, due to demands of shortening a warm-up time and saving energy in a fixing unit (see, for example, Japanese Unexamined Patent Publication No. H06-318001). Attention has been also focused on an electromagnetic induction heating method (IH) with a possibility of quick heating and high efficiency heating in recent years, and many products as a combination of electromagnetic induction heating and the employment of a belt have commercialized in light of saving energy upon fixing a color image. In the case of combining the employment of a belt and electromagnetic induction heating, an electromagnetic induction device is often arranged at an outer side of the belt due to merits that a coil can be easily laid out and cooled and further the belt can be directly heated (so-called external IH).
In the above electromagnetic induction heating method, various technologies have been developed to prevent an excessive temperature increase in a paper non-passage area in consideration of a sheet width (paper width) passed through the fixing unit. Particularly, the following prior arts are known as size switching means in the external IH.
An apparatus of a first prior art (Japanese Unexamined Patent Publication No. 2006-163200) has a function of displacing magnetic shielding members arranged between an induction heating coil and a core between a magnetic path shielding position located at a coil center and a magnetic path releasing position located at a wound part of the coil. According to the first prior art, if the magnetic shielding members are displaced from the wound part of the coil to the coil center, a magnetic path corresponding to a paper non-passage area of a heating roller is shielded, wherefore an excessive temperature increase in the paper non-passage area of the heating roller can be prevented by a small-size construction.
An apparatus of a second prior art (Japanese Unexamined Patent Publication No. 2006-267180) shields magnetism by using a magnetic shielding plate having high magnetic permeability and high electrical resistance. Particularly, according to the second embodiment, the magnetic shielding plate is normally located outside a gap between a fixing roller and a magnetic flux generator, but moves into the gap as a thermal actuator is deformed upon reaching a high temperature.
However, the magnetic shielding members used in the first prior art are aluminum plates having a certain wide area. Even if these aluminum plates are moved to the magnetism releasing position, they overlap with the coil, which exhibits a certain magnetic shielding effect. Therefore, the first prior art has a problem of deteriorating heat transfer efficiency during heating.
On the other hand, the second prior art has no problem of exhibiting the magnetic shielding effect when the magnetic shielding plate is retracted as described above since the magnetic shielding plate is retracted to the outside of the gap. However, a storage space is necessary upon retracting the magnetic shielding plate to the outside of the gap, which causes another problem of forcing an installation space for the coil and the like to be reduced by that much. Therefore, according to the second prior art, it is difficult to ensure a sufficient installation area for the coil for the fixing roller and there is a problem of leading to a corresponding reduction in heating efficiency. In order to compensate for this, the fixing roller itself needs to be enlarged. However, since the enlargement of the fixing roller leads to an increase in heat capacity, it is unfavorable in shortening a warm-up time.